In recent year there has evolved a demand for in process high precision measurements. For many applications, the measuring process should preferably be fast, such that it can be used for continuous controlling of e.g. a milling machine or an EDM (electrical discharge machine).
The above is illustrated by the following example. In an EDM, a chuck and a holder is normally used to clamp the work piece that is to be machined. The position accuracy in the plane of the chuck is normally about +/−2 μm, and this is sufficient for most application. However, a major application area is manufacturing of electrodes for EDM:ing (Electrical Discharge Machining) and mould making by EDM:ing, as the last step. In recent years there has been an upcoming need for high precisions moulds, which are used for production of e.g. optical lenses with very high accuracy and very accurate micro parts. Further, there is also a need for extremely accurate machining of micro parts. The position accuracy for the coupling must then be on sub-micron level (e.g. +/−0.2 μm). This is not possible to achieve with conventional couplings and especially not in the environment of an electrode and mould production process, as conventional system often are space needing and sensitive to traces of liquids, such as dielectricum and cooling liquids, fog, smoke, dust and particles from the process. Moreover, the sub-micron position of the holder is affected by the process forces and temperature gradients. Continuous position measurements with sub-micron accuracy are therefore preferably used.
Further, the chuck is subjected to wear, which depends on the number of clampings, the process forces and particles from the process. The wear affects the position accuracy and must be monitored to replace the chuck in time before it has been worn down. Normally, the loss of accuracy due to wear is today measured in a separate process, often manual, in stead of at each clamping.
U.S. Pat. No. 6,078,706 discloses a quasi-static fiber pressure sensor using self-referenced interferometry based on a broadband semiconductor source which probes the pressure plate deflection within a Fabry-Perot cavity where phase is demodulated with a dual grating spectrometer providing real-time, high resolution remote measurement of pressure using optical interrogation of a deflecting pressure plate. This technique yields absolute gap measurement in real time over a wide range of gap lengths with nanometre resolution. By tailoring the pressure plate design to cover the range of gaps and deflection that can be resolved, pressure sensing with psi resolution can be obtained in a kpsig pressure range.
U.S. Pat. No. 7,099,015 B2 describes a further fiber optic sensing device, which uses a Fabry-Perot cavity to sense a physical parameter. The cavity modulates the incident polychromatic light. The modulated light is recorded by an optical spectrometer means. The spectrum is analyzed in a signal processing unit which normalizes the spectrum and determines the phase of the modulated signal. The phase accumulated over whole range of wavelengths, has been used for identification of the physical parameter using a look-up-table. The cavity, the polychromatic light source and the spectroscope means are connected by fiber optic means.
Egrov, S A: “Spectral signal processing in intrinsic interferometric sensors based on birefringent polarization-maintaining optical fibers”, Journal of lightwave technology, Vol. 13, No. 7, July 1995, pages 1231-1236, ISSN: 0733-8724, which was cited against EP 08 166 344 wherefrom this application claims priority, describes a spectral signal processing technique which is applied in intrinsic strain, temperature and distributed linear position interferometric fiber optic sensors based on birefringent polarization maintaining fibers. The method provides non-incremental measurements of external physical parameters affecting the sensing fiber. Simultaneous interrogation of sensors in a network is also discussed.
According to this article an accurate value of the optical path difference is calculated according to the shift of one spectral resonance, with a known interference order, in the boundaries of one free spectral range from a predetermined frequency. Further, the article underlines that a prerequisite for the described signal processing algorithm is an unambiguous determination of the interference order. Moreover, the article explains that for an unambiguous determination of the interference order, it is required that the variance of the measurement of the optical path length is less than one sixth of the wavelength. Hence, it is a limitation that the algorithm is adapted for measuring small length differences, and also that the algorithm requires an unambiguous determination of the interference order.
Recently, an optical setup has been presented which enables position measurements with improved accuracy. The setup is described in EP 1 849 556, which is hereby incorporated by reference. According to one embodiment, the chuck presented therein has embedded position sensors for measuring six degrees of freedom (X, Y, Z, Xrot, Yrot and Zrot). The location of the sensors and the cross like beam structure of the holder allows the position deviations to be transformed to e.g. forces and moments in all directions (Fx, Fy, Fz, Mx, My, Mz). The position and load information may be used for monitoring and adaptive control purposes.
EP 1 849 556 refers to DE 195 28 676, which describes an optical system and a method of absolute distance measurements using two lasers of different frequencies. According to the measurement principle of DE 195 28 676 an interferometer is used, wherein the light from one of the lasers is sent in one leg of the interferometer and reflected towards the object surface, the distance to which is to be determined. The light from the other laser is sent in the other leg of the interferometer and reflected towards a reference surface, the distance to which is known. The two reflected light beams are super positioned, and by continuously changing the frequency of one of the lasers a varying intermediate frequency is formed. This intermediate frequency contains all of the necessary phase information required for distance measurements. The periodic signals are proportional to the change in frequency of the laser, as well as to the path difference of the interferometer.
A disadvantage of the above described method is that the optical signals received from the reference object must be continuously compared to the signals from the reference interferometer in order to enable a determination of the distance to the object. Hence, an age variation in the reference interferometer may cause a change in the measurement results. Further, due to the rather large number of components the system is space consuming. Additionally, as the measurement result is partially determined by the path difference between the two legs of the interferometer, the device is sensitive to misalignments.